Brake warning switch with bypass

ABSTRACT

A dual pressure switch unit utilized in dual brake systems for motor vehicles includes a valving mechanism operable by a pressure differential to change the circuitry of one of the two dual systems. In a system utilizing a pressure reducing valve in the rear brakes, failure of the front brakes will operate the valve and switch unit to close the switch to actuate a signal and to bypass the pressure reducing valve to supply full pressure to the rear brakes. This valving action is produced by an opening in a central valve spool normally closed by a member which is restrained in the housing and moves away from a closed position against the opening when the valve member is moved axially by a pressure differential. Whenever the valve member is returned to a neutral position, the opening is closed to return the pressure reducing valve to the circuit.

United States Patent Keady 154] BRAKE WARNING SWITCH WITH BYPASS [72]Inventor: Frederick D. Keady,

Heights, Ohio [73] Assignee: The Weatherhead Company, Cleveland, Ohio[22] Filed: Aug. 7, 1969 [21] Appl. No.: 848,321

Cleveland [52] U.S. CI. ..303/6 C, 188/151 A, 200/82 D, 303/84 A, 340/52C [51] Int. Cl ..B60t 8/26, B60t 11/34, B60t 17/22 [58] Field ofSearch.....303/84, 84 A, 6, 6 C; 200/82, ZOO/82.31; 340/52, 52 C; 92/51;1881151.] 1, 152.11; 137/87,98101; 116/70; 60/54.5

[56] References Cited UNITED STATES PATENTS 3,441,318 4/1969 Bueler..303/6 C 3,448,230 6/1969 Buelen: ..303/6 C X 3,450,443 6/1969 Bueler..303/6 C 3,464,741 9/1969 Falk ..303/6 C [451 Oct. 17,1972

Primary Examiner-George E. A. Halvosa Assistant Examiner-John J.McLaughlin Attorney-McNenny, Farrington, Pearne and Gordon [57] ABSTRACTA dual pressure switch unit utilized in dual brake systems for motorvehicles includes a valving mechanism operable by a pressuredifferential to change the circuitry of one of the two dual systems. Ina system utilizing a pressure reducing valve in the rear brakes, failureof the front brakes will operate the valve and switch unit to close theswitch to actuate a signal and to bypass the pressure reducing valve tosupply full pressure to the rear brakes. This valving action is producedby an opening in a central valve spool normally closed by a member whichis restrained in the housing and moves away from a closed positionagainst the opening when the valve member is moved axially by a pressuredifferential. Whenever the valve member is returned to a neutralposition, the opening is closed to return the pressure reducing valve tothe circuit.

10 Claims, 3 Drawing Figures PATENTEDnm 11 ran BRAKE WARNING SWITCH WITHBYPASS BACKGROUND OF TI'IE INVENTION This invention relates generally todual braking systems for motor vehicles and more particularly to sensingdevices operable to detect a pressure unbalance between the two systemsresulting from a failure in one system and to the incorporation of avalving action in such a device which is able to bypass a proportioningor pressure reducing valve in one of the brake systems to allow fullapplied brake pressure to be transmitted directly to the brakes bybypassing the pressure reducing valve in the event of failure whileretaining the valve in the system when both brake systems arefunctioning in the normal manner.

With the advent of dual braking systems in automobiles, it wasrecognized that the advantages of such a system, that one set of brakeswould remain operative in the event of the failure of the other systemas a result of a leak or other failure, would require that some signalbe provided to warn the driver of such failure. The signal is necessarybecause with the powerful characteristics of brakes used in modern motorvehicles, a driver might not notice the failure by virtue of anyincreased requirement in applied pressure or stroke of travel of thebrake pedal. For these reasons various methods have beendevised forproviding a sensing device, usually in the form of an electrical switch,which is sensitive to a pressure differential between that in the twosystems to operate a warning light or other device operable to warn thedriver of the vehicle that a failure in one of the systems has takenplace. The most common form of such a device is in the form of anelectric switch utilizing a movable member exposed to the pressures inboth systems such that the switch remains inoperative as long as the twosystems are in substantial balance. However, when a pressure imbalanceoccurs between the two systems, as a result of a failure or leak in oneof the systems, the device responds by closing an electric circuit tooperate a warning device such as a signal light on the dashboard. lnbrake systems where certain valves are used in one half of the system tomodify the pressure between that coming from the master cylinder andthat applied directly to the brake operating slave cylinders, it hasbeen recognized that the pressure reduction produced by such valves isundesirable whenever a failure in the other half occurs. A most commonapplication of such a system is in hybrid braking systems using a discbrake arrangement on the front and a selflenergizing type drum brake onthe rear wheels where, because of the different characteristics of thetwo brake units, premature lockup of the rear wheels will occur becauseof excessive pressure unless a proportioning valve is interposed betweenthe master cylinder and the rear brake actuating motors to limit thepressure buildup in the actuating motors below that coming from themaster cylinder whenever the pressure levels rise above a certainpredetermined level. Thus, in such braking systems, while the frontbrakes receive pressure directly proportional to the force applied tothe brake pedal, the rear brakes, before the pressure reaches a certainlevel, also receive a similar pressure, but if the pressure increasesabove that level, they receive a proportionally lower pressure resultingfrom the pressure reduction taking place in the proportioning valve.

In such a system, if the front brakes fail, there will be an increasedpressure requirement in the rear brakes to produce the necessary brakingaction. Because of the presence of the proportioning valve in such asystem, an even greater force is required at the brake pedal than wouldotherwise be necessary to actuate the rear brakes if they received thefull pressure. Therefore, it has been recognized as desirable in suchcases that the proportioning valve in the rear brake system be bypassedupon a failure of the front brakes so that the rear brakes receive thefull pressure from the master cylinder regardless of the pressure levelin the system. It has been recognized that the brake failure sensingdevice which detects the pressure loss in the front system and moves thevalve member accordingly is suitable because of its existingconstruction to cause this valving action. l'leretofore, it has beenproposed that the moving valve or switch member, usually in the form ofa spool sliding in an axial bore, include a valving land constructionadapted to move across grooves in the bore in the housing to allow thisbypass to occur through the axial bore so that fluid may flow directlyfrom the master cylinder to the rear brake line by passing theproportioning valve. However, such construction generally required anexcessively long axial movement of the spool as well as that the sealingmember such as an O-ring move across a radial groove in the bore withpossible resulting damage to the 0 ring, so that the failure of the sealat this point would result in the bypassing of the necessaryproportioning valve even when the brake system is functioning in thenormal manner.

Accordingly, it is an object of the present invention to provide in abrake failure warning switch a novel valving device operable to bypass apressure reducing device in one of the brake lines upon the occurranceof a pressure unbalance which gives improved operation and is of simpleand dependable construction.

It is a further object of this invention to provide a bypassingarrangement as set forth in the preceding object which requires aminimal axial movement of the failure sensing portion of the unit toallow compact construction while providing high reliability ofoperation.

It is a further object of this invention to provide a device as setforth in the preceding objects in which the valving occurs by a passagethrough an axial spool which provides the failure sensing action, whichpassage is normally closed by a valve sealing member which is restrainedin a passage in the housing so that upon movement of the sensing memberaway from the valve member, the axial passage to the sensing member isopened to bypass the proportioning valve.

Briefly, the foregoing and other and additional objects and advantagesof the invention are achieved, according to the preferred embodiment ofthe invention, by providing a housing having an axial bore therein inwhich is mounted a sliding spool member. The spool member has a portionof reduced diameter at the center and sealing portions on each side ofthis reduced diameter portion. A switch contact projects radially intothe axial bore adjacent the reduced diameter portion. Normally, thespool is held in the center position by means of springs at each end ofthe spool and one end of the spool is exposed to the pressure toward oneside of the dual braking system such as the front brakes, while theother side is exposed to the pressure at the rear brakes. Normally, thepressure to the rear brakes bypasses the portion at its side of thespool member to go to a proportioning device and the flow from theproportioning device, which normally tends to reduce pressure, enters ata port, sealed from the remainder of the device by suitable seals on thespool, to pass around and across the spool and out to the rear brakes.The spool, in addition to the transverse passage allowing such flow, hasa connected axial passage extending toward the inlet end from the mastercylinder. This passage is normally sealed by a resilient poppet held inplace by a cage mechanism guiding it in the bore and held by thecentering spring which produces the necessary centering action on thespool. The cage is mounted in an enlarged bore portion of the housingand if a failure occurs in the front brakes, the increased pressure ofthe rear brakes causes the spool to move in a direction so that theprojecting switch contact engages the spool to complete the electricalcontact. However, such movement of the spool moves the axial boreportion away from the poppet which is restrained in its enlargecounterbore so that fluid can now flow past the poppet and into theaxial bore to the transverse bore and directly to the rear brakes whilebypassing the pressure reducing or proportioning valve device, whichalthough remaining in the system, becomes inoperative because of thebypassing action.

The foregoing and additional objects and advantages of the invention areachieved in the preferred embodiment of the invention as set forthhereinafter in greater detail and shown in the accompanying drawings, inwhich;

FIG. I is a longitudinal cross-sectional view of the preferredembodiment of the invention;

FIG. 2 is a schematic diagram of a brake system incorporating the deviceof FIG. 1; and

FIG. 3 is a perspective view of the poppet mounting device shown in FIG.l.

Referring to the figures in greater detail, the device shown in FIG. 1includes a housing having an axial bore 11 extending lengthwise therein.At its left end, the axial bore 11 has an enlarged counterbore l2terminating at its outer end in an internally threaded portion 14adapted to receive a suitable hydraulic fitting. A fitting member 15 ismounted with a sealing fit within the counterbore l2 and includes anaxial passage 16 opening into the interior of counterbore 12. Upper andlower radial outlets l7 and 18 are provided from the counterbore 12which terminate at their outer ends in suitable fitting receivingportions as well known in the art. It will be understood that while theaxial end fitting and the passage 16 may be considered the inlet and thepassages 17 and 18 as outlets to the two separate front brakes, sinceall of these passages open commonly into the counterbore 12, they areinterchangeable in function and any one may be used as an inlet with oneor more of the remaining fittings used as an outlet.

in the left end of the axial bore 11 is located a piston member 20carrying an O-ring seal 2] adapted to make sealing contact with thebore. A helical compression spring 22 is mounted between the fitting l5and the piston 20 to produce a biasing force in the righthand directionon the piston 20 for the reasons described in greater detailhereinafter. It will be understood that the construction describedhereinabove is representative of typical devices and otherwise forms noportion of the present invention.

At the righthand end of the piston 20, there is provided an intermediatemember 24 having left and right end portions 25 and 26 respectively ofsubstantial bore diameter adapted to make contact with the bore with asufficiently tight fit as to insure electrical contact between theintermediate member 24 and the housing 10. Between the ends 25 and 26 isprovided a reduced diameter portion 28 of substantially less diameterthan that of the axial bore 11, and on the right end 26 a righthandpiston 30 is slidably mounted in the bore 11 and is provided with an0-ring seal 31 to make sealing contact with the axial bore 11. Anelectrical contact stud 33 projects radially into the axial bore 11through a transverse bore 34 and is carried in an insulated mounting bya suitable mounting assembly 35 mounted in a housing 10 which includes aterminal 37 for contact to the external warning device. It will beunderstood that stud 33 and terminal 37 are electrically insulated fromthe mounting member 35 and the housing 10 so that upon movement of theintermediate member 24 in either direction, the contact of the one ofthe end portions 25 and 26 will result in electrical contact between thestud '33 and the housing 10, to complete an electrical circuit betweenthe housing, which is normally grounded, and the external terminal 37.The foregoing structure is substantially the same as that shown in theU.S. Pat. to Paul J. Miller, No. 3,374,322 granted Mar. I9, 1968, andapart from the disclosure herein forms no partof the present invention.

As seen in FIG. I, the portion of the axial bore I] contacted by theO-ring 31, the housing 10 has an enlarged annular port area 39surrounding the axial bore 11 of sufficient diameter that fluid flowingthrough the enlarged annular port 39 may freely flow around and bypassthe piston 30. Adjacent the enlarged annular port 39 the housing 10 'isprovided with an upper passage 4] and a lower passage 42 connecting withthe enlarged annular port 39 and being provided at their outer ends withsuitable fitting receiving portions for connection to the remainder ofthe brake system as described in greater detail hereinafter.

The piston 30 extends axially beyond the port 39 and is provided with asecond O-ring 43 also adapted to make sealing contact with the axialbore 11. Thus, the O-rings 31 and 43 effectively isolate any fluidwithin the annular port 39 from the remainder of the portions in theaxial bore 11. The piston 30 is provided with a transverse passage 44normally in axial alignment with the housing passages 41 and 42 andopening to the exterior of the piston 30 intermediate the O-ring seals3| and 43. The piston 30 also includes an axial passage 45 extending tothe right from the transverse passage 44 and opening at the axial end 47of piston 30 in a conical valve seat portion 46.

To the right of the piston 30 the axial bore ll opens into an enlargedcounterbore 49 which joins the axial bore II at a shoulder 50. Thecounterbore 49 has a radially extending passage 52 formed in the housing10 and provided at its outer end with a fitting receiving portion for ahydraulic line as previously described. At

its outer end, the counterbore 49 is closed by a fitting member 54 ofsimilar construction to the fitting member and provided with an axialpassage 55 which opens into the threaded fitting receiving portion 56formed on the housing 10. A helical compression spring 57 similar inconstruction to spring 22 engages the fitting member 54 in similarmanner to the engagement between spring 22 and fitting member 15.

At the inner end of the counterbore 49, there is located a poppet 59,preferably formed of a suitable resilient material such as rubber orsynthetic rubber resistant to the action of the fluids used in thesystem, and this poppet 59 includes a rounded sealing end portion 60adapted to be received in sealing contact in the valve seat 46 formed onthe piston 30. The poppet 59 is suitably carried and secured to a rigidcarrier member 61 which is provided with a disc like central portion 62having an aperture 65 therein within which the poppet 59 is received.The carrier 61 has a plurality of guide legs 63, shown, by way ofexample only, as being three in number, which extend axially from thecentral portion 62 and have an external diameter less than that of thecounterbore 49 so as to slide axially freely therein, but greater thanthat of the axial bore 11. The central portion 62 is cut awayintermediate the guide legs 63 as shown at 64 to allow fluid flow pastthe carrier member within the counterbore 49. The poppet 59 also carriesa rigid washer member 66 which serves as an abutment for the spring 57which, as can be seen in FIG. 1, biases the poppet 59 and carrier 61toward the left so that the sealing end 60 closes off the valve seat 46and such contact also biases the piston 30 toward the left in oppositionto the biasing action of the spring 22. Thus, the springs 22 and 57 arearranged to be equal in biasing action when the intermediate member 24is positioned so that the ends 25 and 26 are substantially evenly spacedon either side of the stud 33. Thus, the spring 57 normally provides abiasing action for the piston 30 to maintain the intermediate member 24in the centered position as well as providing a biasing action for thepoppet 59 so that the axial passage 45 and piston 30 are normally closedwhen the pressures at either end of the pistons and 30 are substantiallybalanced.

However, in the event of a pressure unbalance which causes the pistons20 and 30 and intermediate member 24 to move to the left, the carrier 61will, by engagement between the guide legs 63 and the shoulder 50, havelimited movement toward the left so that further movement of the piston30 causes the valve seat 46 to move away from the poppet sealing section60. The fluid within the counterbore 49 may then flow through the axialpassage 45, the transverse portion passage 44 and into the enlargedannular port 39.

The aforegoing operation of the device is best understood in connectionwith the schematic diagram shown in FIG. 2. This arrangement shows thedevice of FIG. 1 in an automotive brake system having a dual or tandemmaster cylinder indicated at 68 which has a front brake line 69 and arear brake line 70 extending therefrom and connected to separate sourcesof fluid pressure within the master cylinder 68 in a manner well knownin the art. The front outlet line 69 may be connected to any one of theports 14, 17 or 18, while one or more of the other ports may beconnected to one or more front outlet lines 72 leading to the frontbrakes of the vehicle. Also, as shown in FIG. 2, a wire 74 may beconnected from the terminal 37 to an indicator lamp 75 as may beprovided on the dashboard of the vehicle, the other side of which lampis connected by suitable means to a battery 76 which is grounded at 77,so that upon electrical connection between the intermediate member 24and the stud 33, the lamp 75 may light to indicate that a pressure drophas occurred in either the front or rear brake line. Also, as shown inFIG. 2, the passage 52 is connected by suitable line 79 to aproportioning valve indicated at 81. This proportioning valve in turn isconnected by a line 82 back to the passage 41, while the other passage42 is connected to the rear brake line 84. The proportioning valve 81may be of any construction well known in the art which provides for apressure reduction across the valve so that the pressure in the rearbrake line 84 is, under certain conditions, reduced below the pressurein the rear brake line 70 from the master cylinder. It will beunderstood that proportioning valve 81 and lines 71 and 82 may be builtintegrally in a single assembly with the housing 10 or may be separate,as is desired, since the particular construction of the proportioningvalve forms no part of the present invention.

With this arrangement, since the poppet S9 normally closes off the axialpassage 45, while the fluid from the master cylinder to the front brakesflows through the line 69 to the fitting 15 into the counterbore 12 andfrom there to one or the other of fittings 17 or 18 to the outlet lines72, the fluid from the rear brake line '70 from the master cylinder hasa somewhat different path. The fluid through line 70 enters thecounterbore 49 through the passage and fitting member 54 and from therenormally flows outward through the passage 52 to the proportioning valve81. Fluid from the proportioning valve 81 returns through line 82 toport 41 and then flows either through passage 44 in piston 30 or aroundthe enlarged annular port 39 to port 42 and hence the rear brake line84. It will be understood that since the pressure from the mastercylinder is applied to its full extent to both ends of the assembly ofthe pistons 20 and 30 and intermediate member 24, these members willalways be in balance, since the axial bore 11 is of uniform diameter, sothat the intermediate member 24 never makes contact with the stud 33.

However, upon a loss of pressure in the front brakes, the pistonassembly moves toward the left so that the right end 26 and intermediatemember 24 moves into electrical contact with the stud 33 to light thelamp 7! in the manner indicated hereinabove. When this is done, thecarrier 61 engages the shoulder 50 so that the movement of the piston 30under the pressure present in the counterbore 49 moves the valve seat 46away from the poppet sealing portion so that now the fluid present atfull pressure from the master cylinder within the counterbore 49 mayflow through the axial passage 45 and from there through the transversepassage 44 into the enlarged annular port 39 from which it flows throughthe passage 42 directly to the rear brakes. Thus, upon loss of pressurein the front brake hydraulic system, the proportioning valve 41 and itsattendant function of pressure reduction is bypassed so that the fullpressure in the rear brake line passes directly to the rear brake line84 to increase the effective operation of the rear brakes which isnecessary to provide maximum effective braking action after failure ofthe front brakes.

It should be noted that with the arrangement shown herein, should thepressure failure in the front brake system be eliminated, the action ofthe springs 22 and 57 causes the assembly of the pistons and to againcenter itself within the axial bore 11 so that as the piston 30 againmoves to the right, the valve seat 46 moves into sealing contact withthe sealing portion 60 of poppet 59 to close off the bypass axialpassage 45 and again restore the flow of fluid to the rear brakesthrough the proportioning valve 81 so that the system again operates inthe normal manner.

What is claimed is:

1. In a dual hydraulic system having first and second lines normallysubject to substantially equal pressures, a sensing unit connected tosaid first and second lines to produce a signal responsive to apredetermined pressure differential between said lines and a pressurechanging device in one of said lines for changing the pressure in saidone line, the improvement comprising said sensing unit including ahousing, a first member axially movable in said housing responsive tosaid predetermined pressure differential and having a neutral positionwhen said pressures are balanced, a second member in said housing, saidfirst and second members being constructed and arranged to providerelative movement between said first and second members by saidpredetermined pressure differential, cooperating valve means carried bysaid first and second members and operable by said axial movement ofsaid first member from said neutral position to render said pressurechanging device inoperable to change said pressure in said one line,said cooperating valve means including an internal axial passageextending from an end face of one of said members and a poppet axiallysealingly engaging a portion of said end face surrounding said passagewhen said first member is in said neutral position, and said poppetbeing axially separated from said portion of said end face surroundingsaid passage by said axial movement of said first member from saidneutral position to open said axial passage.

2. A device as set forth in claim 1 including spring means operable insaid neutral position to axially bias said cooperating valve means intoaxially sealing engagement.

3. A device as set forth in claim I wherein said internal axial passageis in said first member.

4. A device as set forth in claim 1 including spring means operable insaid neutral position to axially bias said poppet against said portionof said end face surrounding said passage when said first member is insaid neutral position.

5. A device as set forth in claim 4 including stop means for limitingaxial movement of said poppet toward said portion of said end facesurrounding said passage to permit said separation of said poppet fromsaid portion of said end face surrounding said passage by said axialmovement of said first member from said neutral position.

6. A combination hydraulic brake warning device for use in dual brakesystems including a housing, an axial bore in said housing, piston meansslidably mounted in sai ax'al bore and akin sealin en wa ls ereof,signiii sens ing means ffi fi f grii'ai movement of said piston means insaid bore in either direction from a neutral position, passage means atone end of said axial bore to admit fluid pressure therein, from a firstbrake line, a chamber at the other end of said axial bore at the otherend of said piston means whereby said piston means is normally held inthe centered position by a balanced fluid pressure at said passage meansand said chamber, an inlet to said chamber connected to a second brakeline, said chamber having an outlet passage, a pressure changing valvehaving an inlet and an outlet, said valve inlet being connected to saidchamber outlet passage, a port in said housing intermediate the ends ofsaid piston means, said port being connected to the outlet from saidpressure changing valve and to the brake actuators, an internal axiallyextending passage in said piston means extending from an end face ofsaid piston means in said chamber to said port, and a valve poppet insaid chamber operable to axially engage a portion of said end facesurrounding said passage-to close said passage when said piston means isin a centered position, and said axial movement of said piston means inone direction away from said centered position axially separating saidpoppet from said portion of said end face surrounding said passage toopen said passage to bypass said pressure changing valve.

7. A brake warning device as set forth in claim 6 including stop meansfor limiting axial movement of said poppet toward said portion of saidend face surrounding said passage to permit said separation of saidpoppet from said portion of said end face surrounding said passage bysaid axial movement of said first member from said neutral position.

8. A brake warning device as set forth in claim 7 wherein said deviceincludes a first spring at said one end of said axial bore biasing saidpiston means toward said chamber and a second spring in said chamberbiasing said piston means away from said chamber, whereby both of saidsprings bias said piston means toward said centered position, saidsecond spring also engaging said poppet to hold said poppet in axialengagement with said portion of said end face surrounding said pistonmeans when said piston means is in said centered position.

9. A brake warning device as set forth in claim 8 wherein said stopmeans is an annular shoulder in said housing engageable by said poppetto limit movement of said poppet away from said chamber.

10. A brake warning device as set forth in claim 9 wherein said poppetcomprises a rigid carrier in said chamber adapted to engage said annularshoulder to limit movement of said poppet away from said chamber and anelastomeric poppet member secured to said carrier and adapted to makeaxial sealing engagement with said portion of said end face surroundingsaid piston means.

i i l

1. In a dual hydraulic system having first and second lines normallysubject to substantially equal pressures, a sensing unit connected tosaid first and second lines to produce a signal responsive to apredetermined pressure differential between said lines and a pressurechanging device in one of said lines for changing the pressure in saidone line, the improvement comprising said sensing unit including ahousing, a first member axially movable in said housing responsive tosaid predetermined pressure differential and having a neutral positionwhen said pressures are balanced, a second member in said housing, saidfirst and second members being constructed and arranged to providerelative movement between said first and second members by saidpredetermined pressure differential, cooperating valve means carried bysaid first and second members and operable by said axial movement ofsaid first member from said neutral position to render said pressurechanging device inoperable to change said pressure in said one line,said cooperating valve means including an internal axial passageextending from an end face of one of said members and a poppet axiallysealingly engaging a portion of said end face surrounding said passagewhen said first member is in said neutral position, and said poppetbeing axially separated from said portion of said end face surroundingsaid passage by said axial movement of said first member from saidneutral position to open said axial passage.
 2. A device as set forth inclaim 1 including spring means operable in said neutral position toaxially bias said cooperating valve means into axially sealingengagement.
 3. A device as set forth in claim 1 wherein said internalaxial passage is in said first member.
 4. A device as set forth in claim1 including spring means operable in said neutral position to axiallybias said poppet against said portion of said end face surrounding saidpassage when said first member is in said neutral position.
 5. A deviceas set forth in claim 4 including stop means for limiting axial movementof said poppet toward said portion of said end face surrounding saidpassage to permit said separation of said poppet from said portion ofsaid end face surrounding said passage by said axial movement of saidfirst member from said neutral position.
 6. A combination hydraulicbrake warning device for use in dual brake systems including a housing,an axial bore in said housing, piston means slidably mounted in saidaxial bore and making sealing engagement with the walls thereof, signalsensing means operable by axial movement of said piston means in saidbore in either direction from a neutral position, passage means at oneend of said axial bore to admit fluid pressure therein, from a firstbrake line, a chamber at the other end of said axial bore at the otherend of said piston means whereby said piston means is normally held inthe centered position by a balanced fluid pressure at said passage meansand said chamber, an inlet to said chamber connected to a second brakeline, said chamber having an outlet passage, a pressure changing valvehaving an inlet and an outlet, said valve inlet being connected to saidchamber outlet passage, a port in said housing intermediate the ends ofsaid piston means, said port being connected to the outlet from saidpressure changing valve and to the brake actuators, an internal axiallyextending passage in said piston means extending from an end face ofsaid piston means in said chamber to said port, and a valve poppet insaid chamber operable to axially engage a portion of said end facesurrounding said passage to close said passage when said piston means isin a centered position, and said axial movement of said piston means inone direction away from said centered position axially separating saidpoppet from said portion of said end face surrounding said passage toopen said passage to bypass said pressure changing valve.
 7. A brakewarning device as set forth in claim 6 including stop means for limitingaxial movement of said poppet toward said portion of said end facesurrounding said passage to permit said separation of said poppet fromsaid portion of said end face surrounding said passage by said axialmovement of said first member from said neutral position.
 8. A brakewarning device as set forth in claim 7 wherein said device includes afirst spring at said one end of said axial bore biasing said pistonmeans toward said chamber and a second spring in said chamber biasingsaid piston means away from said chamber, whereby both of said springsbias said piston means toward said centered position, said second springalso engaging said poppet to hold said poppet in axial engagement withsaid portion of said end face surrounding said piston means when saidpiston means is in said centered position.
 9. A brake warning device asset forth in claim 8 wherein said stop means is an annular shoulder insaid housing engageable by said poppet to limit movement of said poppetaway from said chamber.
 10. A brake warning device as set forth in claim9 wherein said poppet comprises a rigid carrier in said chamber adaptedto engage saId annular shoulder to limit movement of said poppet awayfrom said chamber and an elastomeric poppet member secured to saidcarrier and adapted to make axial sealing engagement with said portionof said end face surrounding said piston means.